The present invention relates to a pump, a valve assembly for the pump and a method for facilitating opening a valve in a pump. The present invention particularly, but not exclusively relates to a mud pump, cement pump and a method for facilitating opening a discharge valve in a mud pump or cement pump.
In the drilling of a borehole in the construction of an oil or gas well, a drill bit is arranged on the end of a drill string, which is rotated to bore the borehole through a formation. A drilling fluid known as “drilling mud” is pumped through the drill string to the drill bit to lubricate the drill bit. The drilling mud is also used to carry the cuttings produced by the drill bit and other solids to the surface through an annulus formed between the drill string and the borehole. The density of the drilling mud is closely controlled to inhibit the borehole from collapse and to ensure that drilling is carried out optimally. The density of the drilling mud affects the rate of penetration of the drill bit. By adjusting the density of the drilling mud, the rate of penetration changes at the possible detriment of collapsing the borehole. The drilling mud may also carry lost circulation materials for sealing porous sections of the borehole. The acidity of the drilling mud may also be adjusted according to the type of formation strata being drilled through. The drilling mud contains inter alia expensive synthetic water or oil-based lubricants and it is normal therefore to recover and re-use the used drilling mud, but this requires inter alia the solids to be removed from the drilling mud. This is achieved by processing the returned drilling mud. The returned drilling mud flows from a wellhead located at a top of the well through a flow line. The first part of the process is to separate the solids from the solids laden drilling mud. This is at least partly achieved by flowing the returned drilling mud on to a screen of a vibratory separator to screen the returned drilling mud of large solids. The screened drilling mud flows into a series of partitioned sections in an active mud tank. Further processing equipment such as centrifuges and hydrocyclones may be used to further clean the mud of solids. Each piece of further processing equipment is located on top of the respective portioned section of the active mud tank, drawing pre-processed mud from the previous portioned section and outputting processed drilling mud into the partitioned section therebelow. The solids are: disposed of; cleaned and used as aggregate or the like; or certain solids, such as Lost Circulation Material is returned to clean drilling mud. It is not uncommon to have 30 to 100 m3 of drilling fluid in circulation in a borehole.
The clean drilling mud is pumped into an additions unit of the active mud tank. Additives, such as weighting agents, viscosity control agents and lost circulation material are added to the clean drilling mud in the additions unit. The prepared drilling mud is now pumped into further testing unit of the active mud system and tested before being pumped into the suction tank of the active mud tank.
A mud pump is then used to pump the prepared and tested drilling mud from the suction tank into a top of the drill string to circulate drilling mud through the drill string to the drill bit and back to the surface through the annulus. The mud pump comprises a pumping section and a power section. The power section may comprise an electric motor, hydraulic motor or the like driving a reciprocating piston in the pumping section. The prepared and tested drilling mud flows from the suction tank through a supply hose or pipe, through an inlet valve in the pumping section of the mud pump into a cavity and is pushed out of the cavity through a discharge valve by action of the reciprocating piston into a further pipe or hose which leads to a goose neck on top of a top drive or swivel and into the top of a string of drill pipe extending into the borehole.
The borehole can be several kilometres long and several kilometres deep, so the mud pump is typically able to discharge drilling mud at between 200 bar and 1000 bar at between 300 and 4800 litres per minute.
The mud pump may comprise dual acting reciprocating pistons or single action. The mud pump may comprise a duplex reciprocating piston, triplex piston, or any other number of pistons. It is advantageous to maintain a constant flow of drilling mud through the drill string at a constant pressure. Surges in the flow of drilling mud and surges in pressure may cause problems downhole. Such problems may include: the constant return of drilled solids; build up of solids in the annulus; maintaining pressure in the well to inhibit collapse of the borehole; and maintaining a constant weight of mud in the well.
The flow of drilling mud through the drill string traditionally stops and starts when a section of drill pipe is added or removed from the string of drill pipe during tripping and drilling. However, continuous circulation systems while tripping and continuous circulation systems while drilling are becoming more popular. With these systems, flow of drilling mud downhole is more continuous and pressure can be maintained at a constant level more easily.
Mud pumps are used on land rigs and offshore rigs. Mud pumps are generally located on or beside a land rig or on an offshore drilling platform. However, the mud pump may be located on the sea bed or between the seabed and a surface of the sea.
In deep wells or wells in formations which may easily collapse, it is common to line the borehole with casing. The casing is hung from a wellhead. Similarly, liner is used to case a borehole, except that the liner is hung from the bottom of an existing casing. The casing or liner is cemented in place by pumping cement down through the casing and up through an annulus between the casing and the borehole. To and bottom plugs are used to facilitate the cementing operation. A cement pump may be used to flow cement into the well and then flow of drilling mud or water behind a bottom plug may be used to push the bottom plug down and force the cement up through the annulus. The mud pump may be used to pump the drilling mud or water behind the bottom plug.
A cement pump may also be used in the general construction industry in pumping cement for: below ground structures, such as foundations; above ground structures such as car parks, housing, commercial buildings and sky scrappers; and in the marine industry in construction above and below water structures, such as quays and oil platforms.